Fast Multi-tone Phase Calibration Signal Extraction
This paper discusses a fast algorithm for analyzing the phases of a multi-tone phase calibration signal. The multi-tone method is superior to its single-tone counterpart. The PCal signal is a wide-band frequency comb derived from an ultra-stable frequency standard which is then coupled into the input stage of the radio astronomy receiver chain. The frequency comb allows to determine the signal delay, frequency response and phase shifts induced by the radio telescope instrumentation. The instrumentation delay is one of the several delay components that have to be compensated for in multi-telescope aperture synthesis. The frequency response also helps to monitor the instrumentation. To extract the information from the digitized baseband output of the receiver chain calls for fast numerical methods. A simple fast algorithm is presented in this paper. Two implementations are provided. One is based on precomputed look-up tables. The other is based on fast digital sinewave generators. Both lend themselves to parallel software processing on multi-core and SIMD-capable platforms as well as processing with FPGA gateware. We present optimized reference implementations for Intel/AMD, IBM Cell and nVidia GPU platforms.